In electrophotography, electrostatic recording, and electrostatic printing, an image is formed by a series of processes including forming an electrostatic latent image on an electrostatic latent image bearing member (hereinafter “photoreceptor”), developing the electrostatic latent image into a toner image by a developer, transferring the toner image onto a recording medium such as paper, and fixing the toner image on the recording medium. Developers for developing electrostatic latent images into toner images are of two types: one-component developers consisting of a magnetic or non-magnetic toner and two-component developer consisting of a toner and a carrier.
Owing to its high energy efficiency, a fixing method in which a heat roller directly presses a toner image against a recording medium is widely used. (This method is hereinafter referred to as heat roller method.) Because a great amount of electric power is consumed in the heat roller method, there have been various attempts to reduce electric power consumption. For example, one approach involves reducing output of a heater that heats the heat roller while image formation is not occurring and increasing output of the heater while image formation is occurring. This approach has been widely employed. However, it requires several ten seconds until the heat roller is recovered from the sleep mode and heated to a proper temperature to be ready for fixing, which may be stressful for users. More preferably, the heater should be completely off while image formation is not occurring to more reduce electric power consumption. On the other hand, toners are required to be fixable at much lower temperatures to more reduce electric power consumption.
Because toners are required to have low-temperature fixability and storage stability (i.e., blocking resistance) in accordance with recent developments in electrophotographic technologies, polyester resins are more widely employed as binder resin than conventionally-used styrene resins recently. Polyester resins generally have high affinity for recording media, and therefore they can be fixed on recording media at low temperatures (hereinafter “low-temperature fixability”). For example, Japanese Patent Application Publication No. 2004-245854 describes a toner including a linear polyester resin having specific properties, and Japanese Patent Application Publication No. 04-70765 describes a toner including a nonlinear cross-linked polyester resin obtained from an acid and rosin.
However, these toners still do not meet the requirements of energy saving because they cannot keep sufficient fixing strength on recording media when the fixing time period is short or the fixing temperature is low.
Japanese Patent Application Publication No. 2006-208609 describes a toner including a fixing auxiliary component (plasticizer) which is soluble in resins when heated. In this toner, the fixing auxiliary agent exists in the toner forming its crystalline domains thereof. It is described therein that the toner is given both heat-resistant storage stability and low-temperature fixability. Japanese Patent Application Publication Nos. 2009-109971 and 2006-337872 each describe a toner including a crystalline polyester. It is also described therein that these toners are also given both heat-resistant storage stability and low-temperature fixability.
However, it is likely that such fixing auxiliary components are dissolved in binder resins in the process of manufacturing toner, which may result in poor heat-resistant storage stability of the toner. Recent toners are also required to have high coloring power in view of energy conservation. Thus, recent toners have a high colorant content. However, when a toner containing a fixing auxiliary component (e.g., a plasticizer, a crystalline polyester) has a high colorant content, undesirably, the resulting image has low and nonuniform gloss and poor color reproducibility.
The reason for the low and nonuniform gloss is considered that the colorant dispersed in an amorphous binder resin excessively increases elasticity of the amorphous binder resin. The reason for the poor color reproducibility is considered that the fixing auxiliary component is not compatible with the amorphous binder resin even when heated, and therefore the colorant cannot be uniformly extended over the resulting image.